Polybutylene terephthalate resins are widely used as typical engineering plastics.
Most of these resins, however, are exclusively used for injection moldings, and are rarely used for extrusion moldings, and particularly for extrusion-molding of films. The reason for this is that molding such resin into films is extremely difficult.
Generally, there are two types of methods for production of films, namely, the T-die method and the inflation method. The T-die method is usually such that a stream of molten plastics introduced through one inlet is broadened to a desired film width and made as uniform in thickness as possible, so that the melt in extruded in a thin film pattern through a slit-shaped nozzle, the extruded resin being then cooled into a film. The inflation method is often employed in molding polyethylenes into films, which method generally include extruding the plastic in the form of a tube through an extrusion nozzle, the tube being inflated by blowing a fluid (e.g., air) thereinto, whereby the extruded resin is formed into a tubular film. The T-die and inflation methods have their respective advantages and disadvantages. Generally, the inflation method, as compared with the T-die method, provides higher productivity and greater economy and is suitable for production of thin films, but its applicability varies according to the kind of resin material used. As such, it cannot be said that the method is applicable for use with any kind of resin, and naturally the range of resin varieties with which the method can be employed is limited to a few special kinds of resins, such as polyolefins. Generally, polyesters, such as polyethylene terephthalate and polybutylene terephthalate, are film-moldable by the T-die method, but on the other hand, manufacturing of films of these resins involve various technical problems to be solved and, as such, it has not been industrialized to date.
The present invention is directed to a method which enables films of polybutylene terephthalate to be produced by the inflation technique, and as a result, they found that polybutylene terephthalate could be adapted for film extrusion by the inflation technique by selecting a specific variety of polybutylene terephthalate and a specific range of temperature conditions.